Well, I can help you narrow down your choices some.
Not all of the character meals serve lunch or dinner, so if you wanted to have, say, a mid-afternoon buffet lunch, you may be able to get a meal in at Goofy's Kitchen. Technically, Goofy's serves a breakfast and dinner buffet, but the breakfast hours sometimes extend later into the early afternoon and that can be considered brunch. On those busy days at those times of year, they sometimes close at 2 p.m., and then open up again at 5 p.m. or so for the dinner meal. On the regular days, they close a bit earlier than that, and the characters trickle out. The buffet itself at Goofy's - for breakfast, brunch or dinner - IS huge. It is the biggest buffet out of any of the character meals, I think, and has the widest variety. I love it. It has basically been a staple of my DLR trips since the early '90s.
Ariel's Grotto is open all day long, for all 3 meals, but it is an all-Princess meal. So if Princesses are not your thing, you may want to skip it. Only breakfast, though, is served "family style." Not really a buffet. Lunch and dinner are menu orders and there is no real buffet.
Storytellers Cafe Critter breakfast at the GCH buffet is just that - a breakfast buffet. However, during this time of year, when it is busy, Storytellers has a non-character buffet on weekends for dinner. It is fairly small, but people like it.
Minnie & Friends character meal is also a breakfast buffet only. For all other meals, they serve "buffeteria style," but not buffet.
The new Mickey character meal at the PPH is all-buffet, from what we have been told, but it is breakfast only.
So for a dinner buffet, your only choices would be Goofy's Kitchen or Storytellers's smaller, non-character buffet.